The disclosure of Japanese Patent Application No. 2000-081930 filed on Mar. 23, 2000 including the specification, drawing and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a driving force distribution control apparatus and method of a motor vehicle including a first set of drive wheels and a second set of drive wheels whose driving forces can be respectively controlled.
2. Description of the Background
There are known vehicles each including a first set of drive wheels and a second set of drive wheels whose driving forces can be controlled independently of each other. Examples of such vehicles are four-wheel drive vehicles in which a selected one or ones of an internal combustion engine, an electric motor and a hydraulic motor that function as driving motors is/are provided separately for the front wheels and the rear wheels, and four-wheel drive vehicles in which the driving force transmitted from a single driving motor to the front wheels and the rear wheels is distributed between the front and rear wheels by a torque distribution control clutch.
In the vehicles as described above, the distribution of driving force between the first set of drive wheels and the second set of drive wheels is carried out based on a difference in the rotational speed between the first drive wheels and the second drive wheels. An example of a drive force distribution control apparatus that performs the above function is disclosed in Japanese Laid-open Patent Publication No. SHO 63-203429. The driving force distribution control apparatus as disclosed in this publication is adapted to determine no-load or slipping rotation of a wheel(s) based on a difference between the front wheel rotational speed and the rear wheel rotational speed. Upon detection of such slipping wheel rotation, the apparatus performs distribution of the driving force by reducing the driving force for the drive wheels at which no-load or slipping rotation was detected, and simultaneously increasing the driving force for the drive wheels at which no-load or slipping rotation was not detected. With this arrangement, the driving force of the vehicle is further increased, for example, in the case where the vehicle runs on a road surface with a low road surface friction coefficient, such as an icy road, a compressed-snow road, or the like.
In the aforementioned driving force distribution control apparatus, the difference between the front-wheel rotational speed and the rear-wheel rotational speed used as a basis for control of the distribution of driving force between the drive wheels does not always sufficiently reflect the state (e.g., running conditions) of the vehicle. For example, even if the rotational speed difference is the same, the slip states of the drive wheels may differ to a great extent at different vehicle speeds. If the rotational speed difference is equal to, for example, 50 minxe2x88x921 (rpm), the wheel slip rate and the longitudinal force and lateral force differ between the case where the vehicle speed is 50 km/h and the case where the vehicle speed is 100 km/h, resulting in different degrees of vehicle running stability. Therefore, the aforementioned driving force distribution control apparatus may not be able to ensure a sufficiently high maneuverability.
It is therefore an object of the invention to provide a vehicular driving force distribution control apparatus wherein the vehicle running state is more accurately reflected by the distribution of driving force between drive wheels.
To accomplish the above object and other objects, the invention provides a driving force control apparatus of a motor vehicle including a first set of drive wheels and a second set of drive wheels which are driven with respective driving forces that are controlled independently of each other, which apparatus includes (1) a slip rate difference calculating unit that calculates an actual slip rate difference between a slip rate of the first set of drive wheels and that of the second set of drive wheels, and (2) a driving force distribution control unit that controls distribution of driving force between the first set of wheels and the second set of wheels, based on the slip rate difference calculated by the slip rate difference calculating unit.
In the driving force control apparatus as described above, the driving force distribution control unit controls the distribution of the driving force between the first set of drive wheels and the second set of drive wheels, based on the slip rate difference calculated by the slip rate difference calculating unit. Thus, since the vehicle running state or condition is more accurately reflected by the distribution of the driving force between the drive wheels, the driving force can be more appropriately distributed between the drive wheels, thus assuring a sufficiently high level of vehicle maneuverability.